When it comes to low-budget horror movies, even the good ones tend to slip by unnoticed, which probably explains why “Salem” creators Brannon Braga and Adam Simon were drawn to a brand name like Clive Barker’s “Books of Blood” to package their otherwise generic Hulu horror anthology. Less a proper feature than the ersatz pilot for an open-ended number of future installments, this three-part scare-fest uses the flesh-flaying title chapter in Barker’s 1984 short-story omnibus, “The Book of Blood,” to frame two lesser entries with no relation to the beloved pulp collection — which has already spawned such films as “Rawhead Red,” “Candyman” and “The Midnight Meat Train.”